Generally, a vehicle AC generator is provided with a voltage control unit and the voltage control unit controls an output voltage of the vehicle AC generator to take a target voltage value. In many cases, the voltage control unit is configured to control an output voltage of the vehicle AC generator to take a target voltage value by repetitively giving a control cycle to a power switch element that controls an exciting current flowing through a field coil of the vehicle AC generator and adjusting an excitation duty indicating an ON time ratio of the power switch element in each control cycle.
JP62-64299A discloses gradual excitation control by which an exciting current of a vehicle AC generator is increased gradually with time to suppress an abrupt increase of a drive torque of the vehicle AC generator when a vehicle load is newly activated. Also, JP7-194023A discloses external control by which a target change command for an output voltage of the vehicle AC generator is given to the voltage control unit from an external control unit thereof, so that the voltage control unit adjusts the target voltage value for an output voltage of the vehicle AC generator in response to this target change command with the aim of controlling an output voltage of the vehicle AC generator to take an appropriate value according to an operation state of the vehicle.